For me, there's no rivalry when it comes to websites. We are just giving options to the readers. Readers can use their tabs to view both sites at the same time. It's not like television where you can view one tv shows only in one time. If you will change channel, you will loose the story. Website can be switches through tabs and the idea of the site will not be lost.
Infact, there is the thing called "blogosphere" where bloggers with the same theme are helping and linking each other.